1. Field of the Invention
The present invention relates generally to improvements in kiln temperature adjustment and more particularly pertains to a new and improved method and system for obtaining the temperature profile of a kiln, storing and repeating that profile when desired.
2. Description of the Prior Art
The conventional means of determining the temperature profile of a kiln 11 (FIG. 1) consists of moving a thermocouple 53 through the kiln 11 and recording the temperature sensed by the thermocouple 53 on a strip chart 61 by means of a strip chart recorder 59 to which the thermocouple is connected by wires 57.
This particular method is utilized whether the kiln is a tunnel kiln in which the wares are stacked on a platform 55, for example, or a belt kiln in which the wares move through the tunnel at a specified rate on a conveyor belt, 55, for example. When a belt kiln is utilized, the thermocouple 53 is attached to the belt. If a tunnel kiln is utilized, thermocouple 53 must be pulled through the kiln by some other means, such as on a rod, for example.
A kiln is typically made up of a plurality of zones, 13, 15, 17, 19, 21, 23. Each zone may in turn be made up of a plurality of heating elements. The schematic illustration of the kiln of FIG. 1 illustrates a six zone kiln where the two end zones 13 and 23 are not monitored by controllers and the four middle zones 15, 17, 19 and 21 are monitored by controllers 43, 41, 39 and 37, respectively. Zone 1 controller 37 has a thermocouple 51 located somewhere in its zone 21 which provides the feedback signal to controller 37, telling it to increase or decrease the current supply to the heating elements 33 of its zone. Thermocouple 49 performs the same function for zone 2 controller 39. Thermocouple 47 performs the same function for zone 3 controller 41. Thermocouple 45 performs the same function for zone 4 controller 43. The heating element 25 of end zone 13 and the heating element 35 of end zone 23 are not controlled in this fashion but rather are controlled in an open loop manner.
Heating elements 25, 27, 29, 31, 33 and 35 of their respective zones 13, 15, 17, 19, 21 and 23 can be made up of a single heater unit such as manufactured by the Thermocraft Corporation, for example, or can be made up of a plurality of heater units. The size of the kiln, the number of heating elements and zones used, is determined by the wares and temperature profile to be achieved.
Once the kiln is stabilized, which could take as long as four hours, it is ready to have its temperature profile determined according to the prior art method shown in FIG. 1 by dragging a thermocouple 53 through it. The strip chart recorder 59 records the temperature in the kiln as a function of time as the thermocouple 53 enters the kiln and until it emerges from the exit zone. By knowing the chart speed of the recorder and the belt speed of the kiln, if it is a conveyor kiln or the speed at which it is pulled through, if it is a tunnel kiln, the temperature at any given location in the kiln can be determined.
The trace 67 produced by the strip chart recorder 59 is illustrated in FIG. 2 on strip chart 61. The trace 67 is the temperature profile of the kiln with the temperature 65 being along the Y axis and the distance 63 being along the X axis when the time axis of the strip chart recorder is converted into kiln position. Thus, from the trace 67, it can be seen that the maximum temperature of the kiln occurs at the center and the increase and decrease in temperature along the length of the kiln is shown by curve 67.
There are basically three main factors contributing to the temperature profile of the kiln. The first factor is the settings of the temperature controllers for each controlled zone in the kiln. That is, zones 15, 17, 19 and 21 of the FIG. 1 example. The second factor, which is present in conveyor-type belt kilns is the speed of the belt. Because the belt is relatively massive, its heat capacity causes heat from the entrance of the kiln to be shifted toward the exit as the belt moves through the kiln. If the controlled zones cannot respond rapidly enough to compensate for this redistribution of heat, the temperature at various points throughout the kiln will change causing the profile to also change. A third factor, which is present in all types of kilns is the mass of the ware itself. The ware has a certain heat capacity which will cause the profile of the kiln to change when inserted into the kiln. In a conveyor-type belt kiln, the ware causes the redistribution of heat in much the same way that the conveyor belt does.
In many applications it becomes extremely important that a certain preferred temperature profile be reproduced and maintained during firing of the ware, perhaps because only this profile produces the results desired. In such instances the profile of the kiln must be determined periodically during kiln use, because changes can and do occur in the temperature distribution within even a given zone. This is evident from the fact that each zone is controlled only by one feedback sensor such as a single thermocouple which is placed only at one point within the controlled zone. A change of temperature distribution just in one zone can change the entire temperature profile of the kiln.
The prior art method of moving a thermocouple through the kiln to provide a profile trace by strip chart recorder has a considerable amount of drawbacks, for example:
(1) each time a profile is to be obtained the actual generation of that profile consumes a significant amount of time, some profiles can take up to 12 hours. During the time that a kiln profile is being obtained, no change can be made in the control of the belt speed or current settings to the heating elements without invalidating the profile.
(2) the accuracy to which a particular given desired profile can be reproduced again and again is determined mostly by the skill of the operator, the number of controlled zones available in the kiln, their accuracy of control, and the amount of time available for obtaining a temperature profile.
The present invention does not have any of these drawbacks.